The invention relates to an arrangement for cooling a flow-passage wall surrounding a flow passage, having at least one rib element which induces flow vortices in a flow medium passing through the flow passage.
In the field of gas turbine technology, great efforts are made to increase the efficiency of such plants. It is known that a temperature increase in the hot gases produced by the combustion of an air/fuel mixture inside the combustion chamber is at the same time associated with an increase in the gas-turbine efficiency. However, an increase in the process temperature requires all of those plant components which come into direct thermal contact with the hot gases to have a high heat resistance. However, the heat resistance, even in the case of especially heat-resistant materials, is limited toward the top of the temperature scale, so that melting of the material is unavoidable if certain limit temperatures specific to the material are exceeded. In order to avoid such melting actions and yet ensure high process temperatures inside the gas-turbine system, cooling systems are known which specifically cool those plant components which are directly exposed to the hot gases. Thus, for example, the turbine blades, just like the combustion-chamber walls, are combined with cooling passages through which, compared with the temperatures of the hot gases, relatively cold air is fed, this cold air being branched off, for example, from the air compressor stage for cooling purposes. The cooling-air flow flowing through the cooling passages cools the cooling-passage walls and is itself heated by the latter. In order to improve the cooling effect and the heat transfer associated therewith from the cooling-passage walls to the cooling medium, air, measures have been taken which enable the thermal coupling between cooling medium and cooling-passage wall to be optimized. Thus it is known that, by the provision of rib features on the inner wall of the cooling passage, specific turbulent flow portions can be produced within the cooling-medium flow passing through the cooling passage, and these turbulent flow portions have flow components perpendicular to the cooling-passage wall. In this way, the portion of the cooling-medium mass flow which comes into direct thermal contact with the cooling-passage walls is increased decisively, as a result of which the cooling effect is also considerably improved. Thus, by the provision of appropriate rib features along the cooling-passage wall, a so-called secondary flow forms in addition to the main flow flowing through the cooling passage, the flow portions of which secondary flow, as indicated above, have directions of flow which are largely directed perpendicularly to and away from the cooling-passage wall. In particular in the case of rib features which are of rectilinear form and are arranged at an angle to the main flow direction, it has been found that relatively stable and sharply pronounced secondary flow vortices are formed, and these secondary flow vortices lead to increased intermixing of the boundary layer close to the cooling-passage wall, and this increased intermixing enables an increased amount of cold cooling air to pass to the hot cooling-passage walls.
Extensive studies have been carried out in connection with the rib features inside cooling passages and the effect associated therewith on the heat transfer coefficient occurring between the cooling wall and the cooling medium flowing through the cooling passage. In particular, the studies related to the influence which diverse parameters characterizing the rib features exert on the heat transfer coefficient and on the pressure loss associated with the flow over a rib feature, such as, for example, rib height, inclination of the rib flanks or angular orientation of the ribs of rectilinear design relative to the main flow direction, Reynolds and Prandtl number, the aspect ratio of the cooling-passage cross section, or the rotational vortices forming within the flow of the cooling air, to mention just a few parameters. Most optimization efforts with regard to design and arrangement of the rib features inside cooling passages were restricted to the optimization of the rib cross section.
The object of the invention is to develop an arrangement for cooling a flow-passage wall surrounding a flow passage, having at least one rib element which induces flow vortices in a flow medium passing through the flow passage, is attached to that side of the flow-passage wall which faces the flow passage, and the shape and size of which are selected in accordance with a certain heat transfer coefficient and a certain pressure loss caused in the flow medium due to the latter flowing over the rib element, in such a way that the cooling effect of the flow medium passing through the flow passage is to be further increased without at the same time affecting the heat transfer coefficient, which hinders optimization through the shape and size of the rib element, between cooling-passage wall and flow medium and without sustaining an increase in the pressure loss caused by the flow medium flowing over the rib element. With regard to their production, measures increasing the cooling effect are to involve little outlay and low production costs.
According to the invention, the rib element, while largely retaining its original shape and/or size, has contours enlarging its surface facing the flow passage.
Thus the idea according to the invention is based on the optimization of the outer rib contour with the aim of increasing the heat-transferring surface between rib and flow medium, yet the heat transfer coefficient, defined by the rib form, of the rib and the pressure loss, caused by the rib form, in the flow medium are to remain essentially unaffected.
It has thus been recognized that measures which enlarge the surface of the rib element and which largely have no effect on the heat transfer coefficient and the pressure loss caused by the rib element can have a direct and decisive effect on a marked increase in the heat transfer between the cooling-passage wall and the cooling-medium flow passing through the cooling passage. In particular, the generation of secondary vortices, which is due to the rib elements opposed to the cooling-medium flow, at least in its marginal regions, must be left largely unaffected, so that measures enlarging the surfaces can be produced merely by a slight modification to the rib surfaces.